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Full text of "Text Book Of Mechanical Engineering"

298

Rolls for bending Section Bars.

Vertical rolls are often used for long, heavy plates, and are
said to be less expensive in operation, while giving truer finish to
the end of the bent plate. This last is the principal difficulty
with all rolls, the entering edge, to six inches deep, being always
set by bending while hot with wooden hammers. Except for this,
the plates are never heated for rolling, even up to i\ inches in
thickness, for in such cases the radius is proportionately larger.
The weight of plate is eliminated by the vertical method, with less
fear of obliquity of curvature. Long rolls are often slightly bellied
at the centre, to take up spring. For the heavier plates an hydraulic
bender, introduced by Mr. Tweddell, seems very likely to super-
sede rolls. It finishes the plates to a truer circle from end to end,
and there is no limit to plate thickness, or risk of fracture by too
rapid feed. Butt strips can also be bent truly to boiler curve.
The tool is similar in design to the multiple punch in Fig. 2840,
but the girders are placed vertically, and suitable dies inserted
instead of the row of punches.

Plate-straightening rolls are similar in construction, but there
are some four rollers at top, pressed down simultaneously by
connected screws, upon three rollers at bottom, and the plate is
passed through and through till truly plane.

Rolls for Section Bars (Fig. 288) have their axes vertical,
and are placed upon a table A, which is sometimes conveniently
set flush with the ground, with a pit for the gearing. They are
driven by the usual fast and loose pullies F and L with crossed
and open straps for reversal, These actuate a worm and worm
wheel, B and c, and a spur pinion D on the axis of c gears with
wheels G G on the roller shafts. Thus BE are the driving rollers,
and H the bending roller, with a screw j to bring its bearing closer
to the rollers E E, effected by turning the nut K. A ring or angle
bar is shewn bent to a circle with an outward flange—an inward-
flanged ring being obtained by turning all the rollers upside down,
and other sections by special rollers. Finally the ring is removed
and welded with a glut-piece.

V Flanging Presses.—It being always advisable to dimmish
the number of joints in a boiler, the end plate is usually flanged
or bent over at the edge to form a ledge for the shell-plate, while
stiffening itself considerably.